Palladium deactivated

The partial hydrogenation of a 17-ethynyl group over deactivated palladium occurs more readily than the saturation of any other functional group. This is also true of 17-ethynyl carbinols and 17-acetylenic ethers (52). ... 

Further investigations using model compounds showed that the formation of PdCl2 by a reaction with the solvent, as suggested by Brinkmann el al. [30], was also not responsible for the observed rapid deactivation. Palladium leaching after formation of Pd(0) was also excluded by experiments. The authors concluded therefore that the presence of allyl acetate facilitated the decomposition. 

Acetylenic alcohols, usually of propargylic type, are frequently intermediates in the synthesis, and selective reduction of the triple bond to a double bond is desirable. This can be accomplished by carefully controlled catalytic hydrogenation over deactivated palladium [56, 364, 365, 366, 368, 370], by reduction with lithium aluminum hydride [555, 384], zinc [384] and chromous sulfate [795], Such partial reductions were carried out frequently in alcohols in which the triple bonds were conjugated with one or more double bonds [56, 368, 384] and even aromatic rings [795]. 

With most hydrogenation catalysts of the platinum and nickel groups (Sections 4.2.61, p. 459 and 4.2.50, p. 450), a mixture of products is obtained even if an attempt is made to stop the reaction at the half-way stage. An alkene may only be obtained in good yield if particular attention is paid to the selection of a deactivated hydrogenation catalyst. An early, highly effective formulation of a deactivated palladium catalyst is Lindlar s catalyst (Section 4.2.54, p. 453), but palladium-on-barium sulphate in the presence of quinoline, or palladium-on-calcium carbonate, is also recommended. In these cases it is advisable to... 

Fig. 9. Comparison of the IINS spectrum (TFXA, ISIS) of the deactivated palladium catalyst (solid line) and the results from the Wilson GF matrix method analysis of the spectrum (dashed line). The model is shown in the top right-hand corner.

Outside of some occasional catalyst deactivation, palladium catalysts are noted for their general resistance to catalyst poisons. They are also relatively unaffected by chloride and bromide ions and only moderately inhibited by iodide ions which makes palladium quite useful for the hydrogenolysis of carbon-halogen bonds. The most striking example of this inertness to catalyst poisons is illustrated by the hydrogenolysis of methyl cystine to methyl cysteine over palladium (Eqn. 11.13),a reaction that occurs on one of the most powerful of all... 

The reduction of an acyl chloride can be stopped at an aldehyde if a partially deactivated catalyst is used. This reaction is known as the Rosenmund reduction. The catalyst for the Rosenmund reduction is similar to the partially deactivated palladium catalyst used to stop the reduction of an alkyne at a cis alkene (Section 6.8). 

In the first step of the synthesis (the Kiliani portion), the aldose is treated with sodium cyanide and HCl (Section 18.4). Addition of cyanide ion to the carbonyl group creates a new asymmetric carbon. Consequently, two cyanohydrins that differ only in configuration at C-2 are formed. The configurations of the other asymmetric carbons do not change, because no bond to any of the asymmetric carbons is broken during the course of the reaction (Section 5.12). Kiliani went on to hydrolyze the cyanohydrins to aldonic acids (Section 17.18), and Fischer had previously developed a method to convert aldonic acids to aldoses. This reaction sequence was used for many years, but the method currently employed to convert the cyanohydrins to aldoses was developed by Serianni and Barker in 1979 it is referred to as the modified Kiliani-Fischer synthesis. Serianni and Barker reduced the cyanohydrins to imines, using a partially deactivated palladium (on barium sulfate) catalyst so that the imines would not be further reduced to amines. The imines could then be hydrolyzed to aldoses (Section 18.6). 

The C=N bond is reduced to an imine, using a partially deactivated palladium catalyst so that the imine is not further reduced to an amine (Section 7.9). 

Rosenmund reduction reduction of an acyl chloride to an aldehyde by using a deactivated palladium catalyst. 

Hydrogenation with deactivated palladium black CO CHOH... 

In the second step of the Kiliani-Fischer synthesis, the nitrile is partially reduced to an imine using a deactivated palladium catalyst similar to the Lindlar catalyst used to partially reduce alkynes to alkenes. The imine hydrolyzes to form an aldehyde under the reaction conditions. 

Deactivated palladium supported on zeolite can also be regenerated, and the catalyst must also be reactivated before being reused. This is because palladium in the zeolite framework sinters. By treating the catalyst with an excess of ammonium hydroxide solution, however, the agglomerated palladium dissolves to form a tetramine complex. Calcination in air then decomposes the complex and the original palladium distribution and activity are restored. ... 

Alkynes may be partially reduced to c/s-alkenes using hydrogen and a deactivated palladium catalyst and to frons-alkenes using Na/liquid ammonia. 

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